In crosslinked polymers, since movement of the molecular chains is restrained in three-dimensional directions, the polymers are usually more excellent in strength, heat resistance, solvent resistance, and the like than linear polymers of the same type.
Such crosslinked polymers are roughly classified into (i) those each obtained by converting a monomer into a high molecular compound with a crosslinking agent through three-dimensional crosslinking and (ii) those each obtained by intermolecular crosslinking of a linear polymer with a crosslinking agent.
With regard to the crosslinked polymers of (i), most of them are obtained by fabricating a crosslinkable monomer through a reaction of a monomer with a crosslinking agent beforehand and crosslinking the crosslinkable monomer by heating or by means of a catalyst, and representative examples thereof include thermosetting resins such as urea resins, melamine resins, and phenol resins. In all these resins, a crosslinked structure is fabricated by adding an aldehyde compound such as formaldehyde as a crosslinking agent to ureas, melamines, or phenols as a monomer to form a methylol group and subjecting the methylol groups to a condensation reaction.
Moreover, with regard to (ii), crosslinking methods of various liner polymers have been investigated and put into practical use. Of these, a polyvinyl alcohol-based resin (hereinafter polyvinyl alcohol is abbreviated as PVA), which is a water-soluble resin, is a resin to which water resistance can be imparted by crosslinking and thus is one kind of resins where crosslinked structure fabrication is practically extremely important.
In the case of the PVA-based resin, examples wherein a hydroxyl group bonded to the main chain is used for crosslinking are widely known. However, in order to enhance crosslinking efficiency and fabricate a strong crosslinked structure, it is preferable to use a PVA-based resin wherein a highly reactive functional group is introduced into the side chain. For example, a PVA-based resin having an acetoacetyl group in the side chain has been employed in uses where a high water resistance is required.
As crosslinking agents for the acetoacetyl group-containing PVA-based resin (hereinafter, the acetoacetyl group-containing resin is abbreviated as AA-PVA-based resin), various compounds are known. Of these, aldehyde compounds have been widely used in various uses since they are excellent in reactivity with the acetoacetyl group and a crosslinking reaction proceeds at relatively low temperature.
For example, a crosslinked polymer obtained by crosslinking the AA-PVA-based resin with glyoxal which is a dialdehyde compound is suitably used in a surface protecting layer of heat-sensitive recording media (see e.g., Patent Document 1), an adhesive layer between a polarizing film and a protecting film in polarizers (see e.g., Patent Document 2), and the like.
Moreover, in an aqueous emulsion obtained using a polymer containing a repeating structural unit derived from an ethylenically unsaturated monomer as a dispersoid and the AA-PVA-based resin as a dispersant, an example wherein water resistance is imparted to a dry film of the emulsion by the use of glyoxal as a crosslinking agent (see e.g., Patent Document 3).
Furthermore, in recent years, for the purpose of extending a pot life of a mixed aqueous solution of the AA-PVA-based resin and an aldehyde compound, it is investigated to use a compound whose aldehyde group is protected. For example, there has been proposed a heat-sensitive recording medium wherein an acetal compound obtained by protecting an aldehyde group with a polyhydric alcohol such as glucose is used as a crosslinking agent for the AA-PVA-based resin (see e.g., Patent Document 4).    Patent Document 1: JP-A-09-164763    Patent Document 2: JP-A-07-198945    Patent Document 3: JP-A-11-279509    Patent Document 4: JP-A-2004-291519